Re: [PATCH 2/3] tracing: block-able ring_buffer consumer

[Steven Rostedt]

On Sat, 2009-08-29 at 12:21 +0200, Frederic Weisbecker wrote:
> On Thu, Aug 27, 2009 at 11:03:04AM +0800, Lai Jiangshan wrote:
> > 
> > makes consumer side(per_cpu/cpu#/trace_pipe_raw) block-able,
> > which is a TODO in trace.c
> > 
> > Signed-off-by: Lai Jiangshan <laijs@xxxxxxxxxxxxxx>
> > ---
> >
> >  int ring_buffer_print_entry_header(struct trace_seq *s);
> > diff --git a/kernel/timer.c b/kernel/timer.c
> > index 6e712df..79f5596 100644
> > --- a/kernel/timer.c
> > +++ b/kernel/timer.c
> > @@ -39,6 +39,7 @@
> >  #include <linux/kallsyms.h>
> >  #include <linux/perf_counter.h>
> >  #include <linux/sched.h>
> > +#include <linux/ftrace.h>
> >  
> >  #include <asm/uaccess.h>
> >  #include <asm/unistd.h>
> > @@ -1178,6 +1179,7 @@ void update_process_times(int user_tick)
> >  	printk_tick();
> >  	scheduler_tick();
> >  	run_posix_cpu_timers(p);
> > +	tracing_notify();
> 
> 
> 
> Hmm, that looks really not a good idea. The tracing shouldn't ever impact
> the system when it is inactive.
> Especially in such a fast path like the timer interrupt.
> 

Perhaps we should put a trace point there instead. Then we could add a
probe to it (doesn't need to be an event).

	trace_update_process_times() ?


> 
> 
> >  }
> >  
> >  /*
> > diff --git a/kernel/trace/ring_buffer.c b/kernel/trace/ring_buffer.c
> > index f1e1533..db82b38 100644
> > --- a/kernel/trace/ring_buffer.c
> > +++ b/kernel/trace/ring_buffer.c
> > @@ -443,6 +443,7 @@ struct ring_buffer_per_cpu {
> >  	u64				write_stamp;
> >  	u64				read_stamp;
> >  	atomic_t			record_disabled;
> > +	wait_queue_head_t		sleepers;
> 
> 
> That seems a too generic name. May be consumer_queue?

"waiters" is what is usually used.

> 
> 
> >  };
> >  
> >  struct ring_buffer {
> > @@ -999,6 +999,7 @@ rb_allocate_cpu_buffer(struct ring_buffer *buffer, int cpu)
> >  	spin_lock_init(&cpu_buffer->reader_lock);
> >  	lockdep_set_class(&cpu_buffer->reader_lock, buffer->reader_lock_key);
> >  	cpu_buffer->lock = (raw_spinlock_t)__RAW_SPIN_LOCK_UNLOCKED;
> > +	init_waitqueue_head(&cpu_buffer->sleepers);
> >  
> >  	bpage = kzalloc_node(ALIGN(sizeof(*bpage), cache_line_size()),
> >  			    GFP_KERNEL, cpu_to_node(cpu));
> > @@ -3318,6 +3319,77 @@ ring_buffer_read(struct ring_buffer_iter *iter, u64 *ts)
> >  EXPORT_SYMBOL_GPL(ring_buffer_read);
> >  
> >  /**
> > + * ring_buffer_notify - notify the sleepers when there is any available page
> > + * @buffer: The ring buffer.
> > + */
> > +void ring_buffer_notify(struct ring_buffer *buffer)
> > +{
> > +	unsigned long flags;
> > +	struct ring_buffer_per_cpu *cpu_buffer;
> > +
> > +	cpu_buffer = buffer->buffers[smp_processor_id()];
> > +
> > +	if (!spin_trylock_irqsave(&cpu_buffer->reader_lock, flags))
> > +		return;
> > +
> > +	if (waitqueue_active(&cpu_buffer->sleepers)) {
> > +		struct buffer_page *reader_page;
> > +		struct buffer_page *commit_page;
> > +
> > +		reader_page = cpu_buffer->reader_page;
> > +		commit_page = ACCESS_ONCE(cpu_buffer->commit_page);
> 
> 
> ACCESS_ONCE makes sense if you loop, to ensure the value
> is not cached through iteration, but there I'm not sure this is
> useful.

ACCESS_ONCE is fine. Otherwise we may read it again on the check below.
Of course the worse that will happen is we don't wake up on this tick.

> 
> 
> 
> > +
> > +		/*
> > +		 * ring_buffer_notify() is fast path, so we don't use the slow
> > +		 * rb_get_reader_page(cpu_buffer, 1) to detect available pages.
> > +		 */
> > +		if (reader_page == commit_page)
> > +			goto out;
> > +
> > +		if (reader_page->read < rb_page_commit(reader_page)
> > +				|| rb_set_head_page(cpu_buffer) != commit_page)
> 
> 
> 
> This may need a small comment to explain you are checking that the reader
> is not completely consumed.

Heh, it was obvious for me ;-)

-- Steve


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